1. Field of the Invention
A coupling force control system for a fluid coupling equipped with a lockup mechanism, such as a torque convertor of an automatic transmission for an automotive vehicle, typically changes a coupling force to regulate slippage of the fluid coupling to a target amount in a predetermined region of engine operating conditions. The lockup mechanism provides the fluid coupling with an increase in coupling force when slippage, defined as the difference between rotational speeds transmitted to and from the fluid coupling, rises above a predetermined or target amount, and a decrease in coupling force when slippage falls below the predetermined or target amount.
2. Description of Related Art
A coupling force control system of the type referred to above is known from, for example, Japanese Unexamined Patent Publication No. 57-33,253. In such a system, when an accelerator pedal is depressed to release the lockup mechanism while the engine operates in a specific region of operating conditions, it is desirable not to instantaneously release the lockup mechanism, but rather to gradually lower the coupling force of the lockup mechanism at a constant rate so that the lockup mechanism allows slippage to increase gradually as it is uncoupled. Such a gradual increase of slippage in the lockup mechanism suppresses an abrupt increase in the rotational speed of the engine and helps to improve drive feeling.
Nevertheless, in the system mentioned above, in spite of a large displacement of the accelerator pedal, accompanied by an abrupt increase of drive torque, the lockup mechanism is unlocked with a gradual increase of slippage. Consequently, the frictional elements of the lockup mechanism are subjected to excessive wear, and the durability of the lockup mechanism is lowered.